First, please create an account by clicking on Sign Up in the top right corner of the window and follow the instructions to create an user account.
Don’t forget to write down your username and the password! It is a good idea to save these credentials on your Google account, then you have it always available.
A short introduction to p5.js you find here. Whenever you have finished a program or need to stop, don’t forget to save your file by selecting Save in the File menu item of the editor. Later you can reload your project with Open and continue to work.
A more comprehensive source to learn p5.js is Getting Started With p5js. This is a good introductory book with lots of code samples you can copy and paste into the editor and try out. Happy Coding is another site that introduces you to p5.js covering some topics with tutorials, and others with sample code.
These information will allow you to get up and running, on this site you also find more posts on p5.js going into more detail and showcasing sample projects.